A fibre used to guide optical radiation is constituted by two coaxial, refractive media: a core having a high refractive index and a medium surrounding the core and having a lower refractive index, which is referred to as the cladding. The radial variation in refractive index is chosen so that the radiation is totally reflected with the result that it remains confined within the core. In practice, the diameter of the core is generally between some few microns and around one hundred microns, and the diameter of the cladding is therefore in a range somewhat higher than this. For reasons of protection and facility of manipulation, the thus constituted fibre or a group of assembled identical fibres is surrounded by a protective sleeve, generally cylindrical in shape and made of a flexible material; each assembly constitutes a cable.
The object of the present invention is plugs which make it possible to achieve good connection between optical fibres or between optical fibres and an optical device. When two such optical fibres are to be connected, their terminal faces should be placed in the most intimate possible contact, so that the most perfect possible degree of coincidence is achieved. Moreover, their respective axes should be aligned at the level of the zone of interconnection to improve the connection optical efficiency. Consequently, the optical fibres are arranged in plugs equipped with flat terminal faces with which the ends of the optical fibres themselves finish flush; the arrangement is such that the direction of plugging in of the fibres is coincidental with the fibre axis. Since the connectors designed to receive the plugs have sliding surfaces which make it possible to effect assembly without any clearance the precise positioning of the fibres is determined using as a datum sliding surfaces pertaining to a bushing of the plug. Thus, the conditions of centring and alignment are fulfilled.
In the prior art, it is known to position the terminal part of an optical fibre within the body of a cylindrical bushing which is a solid of revolution with a right terminal section. The coaxial centring of the fibre in relation to the annular bushing is achieved by means of a set of six elastic rods having circular cross-sections, which serve as spacers between the fibre and the bushing and have the same diameter as the former. The elasticity of the spacer rods and their small diameter give rise to substantial deformations which are detrimental to the perfect centring of the fibre. Since the size of the rods is very small, accurate machining of them is a delicate operation. Finally, it turns out that there is no point in using six intervening rods when three would be sufficient.
In the same context, it is known to maintain an optical fibre in position by means of a set of three rigid rods; the rods are clamped with a thermo-retractable sleeve or equipped with clipping spring arrangements. This kind of solution leads again to a positioning inaccuracy and requires a high degree of skilfullness for assembling the various parts.
It is also known to connect end to end planar arrangements of optical fibres between two plates containing semicircular projections. Two adjacent projections define a groove which receives two ends of optical fibres which are to be connected. The plates are tied together by a clipping operation, and, since the projections are made of a material which is less rigid than that of the fibres, they deform. Although the end to end fitting of the fibres may be correct, this connecting technique does not ensure accurate location of the fibres in relation to the external faces of the assembly plates.